Energy in the form of electromagnetic radiation or light in frequency ranges extending from the radio frequency (rf) through visible light wavelengths to the x-ray wavelength range is used not only in the field of communications and lighting applications, but also in a wide range of industrial processes. Infra-red (IR) radiation, which is radiation found just beyond the red end of the visible spectrum and having a longer wavelength and lower frequency than visible light, may be used in medical applications such as therapeutic IR heat treatments and in communications systems.
Ultra-violet (UV) radiation, on the other hand, found just beyond the violet end of the visible light spectrum, has a shorter wavelength and higher frequency than IR radiation and is being used increasingly in various industrial, chemical and pharmaceutical applications. By way of example, UV radiation is capable of destroying microbiological organisms and is used for sterilization of medical instruments, toothbrushes and so forth. It also can be utilized to initiate certain photo-activated processes such as the cross-linking of existing polymers with one another in coating or adhesive applications (for example, the setting of epoxy resins in commonly used household and industrial glues) or in the polymerization of monomer molecules in a chemical reaction to form polymer chains or three-dimensional networks.
The wave/particle duality of the nature of light permits it to be characterized in terms of a wave having a wavelength and amplitude, analogous to a wave which may be observed on a body of water. Light may also be treated as a discrete particle of energy known as a photon characterized by its frequency times Planck's constant, h; accordingly, the higher the frequency of light, the higher the quantized energy carried by it.
As applications for electromagnetic energy of various frequencies and wavelengths evolved, it became desirable to convert light from one frequency range and energy level to another. Down conversion is the process of converting higher energy light to lower energy light. Conversely, upconversion is the process of converting lower energy light to higher energy light. Upconversion is typically a multi-photon absorption process where two or more photons are used to promote an excited electronic state in a host medium which, in turn, radiates at a wavelength of light that has a higher energy level than the energy of the incident light employed to initiate the process. Examples of up and down conversion systems and uses therefor are disclosed in U.S. Patent Application Publication No. US 2010/026123 published by Vo-Dinh et al., Oct. 14, 2010, now U.S. Pat. No. 8,389,958 B2, issued Mar. 5, 2013, and in U.S. Reissued Patent No. RE43,944 E issued to Zhang et al., Jan. 29, 2013.
More specifically, the upconversion of lower energy near infrared (NIR) light to higher energy UV light has found application in the curing of certain polymers which are used in surgical, dental and prosthetic devices. See Alexander Stepuk, et al., Use of NIR Light and Upconversion Phosphors in Light-Curable Polymers, Jour. Of Dental materials, 28, pp. 304-311 (2012) and Uo, et al., Preparation and Properties of Dental composite Resin Cured under Near Infrared Irradiation, Jour. Of Photopolymer Science and Technology, 22(5): 551-554 (2009). Both articles discuss the use of additives or dopants to dental composite materials in conjunction with the application of NIR light energy to address the problems encountered in curing such materials using less penetrating UV energy. However, problems of efficiency and cost-effectiveness in practical commercial applications remain, as discussed in greater detail below.
Polymerization is the process of reacting monomer molecules together in a chemical reaction to form polymer chains or three-dimensional networks. Photo-induced polymerization or photopolymerization makes use of electromagnetic energy, i.e., photons, to initiate the polymerization reaction. Any photopolymerization process includes three basic molecular components: a sensitizer/donor, an initiator/acceptor, and a monomer (or pre-polymer). In some material systems, the initiator itself is photo-activatible and a sensitizer is not necessarily required.
The systems disclosed in the prior art using conventional energy transfer mechanism in the process of polymerization by NIR light require that the NIR light be first absorbed by a substrate matrix, typically a material comprised of nanocrystals of a selected composition. Photon-Phonon energy transfer effectively stores the NIR energy in the vibrational modes of the nanocrystals long enough for more NIR photons to arrive and excite the nanocrystals further, effectively integrating the NIR excitation to progressively higher levels. NIR to blue and UV upconversion is known to be a four- and five-photon integration process, respectively. The nanocrystals can emit the total stored energy as blue or UV light which can then be absorbed by the initiator in a process called re-absorption.
Normally, photopolymerization is induced by relatively high-frequency optical radiation such as ultraviolet (UV) and blue light as discussed above, because these wavelengths have sufficient energy to induce molecular changes in common initiators and sensitizers. The problem for UV and blue light is that the monomer penetration depth is wavelength dependent with higher frequencies being absorbed more rapidly. In other words, the UV energy has penetration depths of levels on the order of approximately one millimeter, as is shown in FIG. 1-2, Standard Method of Photopolymerization. Thick photo-cured systems of more than a few hundred microns are typically achieved in a layer-by-layer approach with reapplication of uncured materials at each step. Layer-by-layer deposition is time consuming and has the potential of introducing internal defects in the resultant solid.
Recently, a variety of different molecules and nanoparticles have been synthesized which are capable of converting low-frequency radiation to high-frequency radiation via the upconversion process. As discussed briefly above, upconversion obeys energy conservation because more photons of lower energy are converted to fewer photons of higher energy. In general, the process can be referred to as multi-photon conversion, and different molecules and nanoparticles achieve the effect by totally different linear and non-linear optical means. Upconverting materials suggest a mechanism of utilizing lower energy light, including near infra-red light to induce photopolymerization without modifying industry accepted initiator/monomer systems. Using lower energy light to initiate polymerization is attractive because of the availability of lower-cost light sources and the deeper penetration depth, in particular when using NIR light as opposed to potentially dangerous and eye-damaging UV light.
One increasingly popular approach to upconversion is to use inorganic nanoparticles, or nanocrystals, doped with Rare Earth elements. Sodium Yttrium Fluoride (NaYF4) nanocrystals doped with various other Rare Earth elements such as Erbium, Europium, Ytterbium and Thulium have recently been identified by numerous groups to be capable of converting NIR to red (Europium), green (Ytterbium), blue and UV (Thulium). NaYF4 nanocrystals have the added benefits of being intrinsically colorless and have been shown to be non-toxic in animals and single cells. However, NaYF4 and other potentially useful nanocrystals are not directly miscible with many industry standard monomer systems.
Accordingly, a need exists for a system and method for improving the efficiency of energy transfer-initiated photopolymerization using lower energy light sources and permitting deeper, single operation polymerization processes in commercial applications via the use of stably miscible nanocrystals in a monomer matrix for improved, cost-effective quality control of the end product.